Variability of X-ray polarization of Cyg X-1

TL;DR

This study presents a three-year multiwavelength polarimetric monitoring of Cyg X-1, revealing state-dependent X-ray polarization degrees, their energy dependence, and alignment with the jet, providing insights into the emission mechanisms and geometry.

Contribution

It provides the first detailed measurement of X-ray polarization variability in Cyg X-1 across spectral states, highlighting the evolution of polarization properties and their relation to jet orientation.

Findings

X-ray polarization degree is higher in the hard state (~4%) than in the soft state (~2.2%).

Polarization degree increases with photon energy in both states.

Polarization angle aligns with the radio jet and shows orbital variations in the hard state.

Abstract

We present the results of a three-year X-ray, optical, and radio polarimetric monitoring campaign of the prototypical black hole X-ray binary Cyg X-1, conducted from 2022 to 2024. The X-ray polarization of Cyg X-1 was measured 13 times with the Imaging X-ray Polarimetry Explorer (IXPE), covering both hard and soft spectral states. The X-ray polarization degree (PD) in the hard state was found to be $\approx4.0\%$, roughly twice as high as in the soft state, where it was around $2.2\%$. In both states, a statistically significant increase of PD with the energy was found. Moreover, a linear relation between PD and spectral hardness suggests a gradual and continuous evolution of the polarization properties, rather than an abrupt change of polarization production mechanism between states. The polarization angle (PA) was independent of the spectral state and showed no trend with the photon energy. The X-ray PA is well aligned with the orientation of the radio jet, as well as the optical and radio PAs. We find significant orbital changes of PA in the hard state, which we attribute to scattering of X-ray emission at intrabinary structure. No significant superorbital variability in PD or PA was found at the period $P_{\rm{so}}$ = 294 d. We also find no correlation between the X-ray and optical polarization; if any, there is a long-term anti-correlation between the X-ray PD and the radio PD.